A DOAS-like method for total column of CO₂ from ground-based FTS measurements of the direct solar beam

Abstract. A new algorithm, similar to that of DOAS method, is developed to retrieve the total column CO₂ from the ground-based hyper-spectral measurements of direct solar beam. Other than spectrum fitting method, which is generally used in the optimal estimation algorithm, the radiances ratio at two wavelength named channel pair, where one is of weak and the other is of relatively strong absorption, is used to retrieve the total column CO₂ in the Short Wave InfraRed(SWIR) band. Sensitivity studies show that this DOAS-like method is less dependent on the model parameters such as aerosols, water vapor, surface pressure, temperature, wavelength shift and signal noise, and the pairs of channels are carefully selected based on the sensitivity studies. To validate the algorithm, the FTS measurements located at Xi'Chong astronomical observatory are used to derive the total column CO₂ amount, 272 pairs ratios are used in the retrieval and the results agree very well with that of GOSAT, which shows that the DOAS-like method could give reasonable value of X_CO2.